Confirmation of a meteoritic component in impact‐melt rocks of the Chesapeake Bay impact structure,Virginia, USA—Evidence from osmium isotopic and PGE systematics     

Seung Ryeol Lee  J. Wright Horton  Richard J. Walker 《Meteoritics & planetary science》2006,41(6):819-833

Abstract— The osmium isotope ratios and platinum‐group element (PGE) concentrations of impact‐melt rocks in the Chesapeake Bay impact structure were determined. The impact‐melt rocks come from the cored part of a lower‐crater section of suevitic crystalline‐clast breccia in an 823 m scientific test hole over the central uplift at Cape Charles, Virginia. The ¹⁸⁷Os/¹⁸⁸Os ratios of impact‐melt rocks range from 0.151 to 0.518. The rhenium and platinum‐group element (PGE) concentrations of these rocks are 30–270x higher than concentrations in basement gneiss, and together with the osmium isotopes indicate a substantial meteoritic component in some impact‐melt rocks. Because the PGE abundances in the impact‐melt rocks are dominated by the target materials, interelemental ratios of the impact‐melt rocks are highly variable and nonchondritic. The chemical nature of the projectile for the Chesapeake Bay impact structure cannot be constrained at this time. Model mixing calculations between chondritic and crustal components suggest that most impact‐melt rocks include a bulk meteoritic component of 0.01–0.1% by mass. Several impact‐melt rocks with lowest initial ¹⁸⁷Os/¹⁸⁸Os ratios and the highest osmium concentrations could have been produced by additions of 0.1%–0.2% of a meteoritic component. In these samples, as much as 70% of the total Os may be of meteoritic origin. At the calculated proportions of a meteoritic component (0.01–0.1% by mass), no mixtures of the investigated target rocks and sediments can reproduce the observed PGE abundances of the impact‐melt rocks, suggesting that other PGE enrichment processes operated along with the meteoritic contamination. Possible explanations are 1) participation of unsampled target materials with high PGE abundances in the impact‐melt rocks, and 2) variable fractionations of PGE during syn‐ to post‐impact events.  相似文献   

Numerical simulations of weak lensing measurements     

David J. Bacon  Alexandre Refregier  Douglas Clowe  Richard S. Ellis 《Monthly notices of the Royal Astronomical Society》2001,325(3):1065-1074

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Rapid vegetation change during the early Holocene in the Faroe Islands detected in terrestrial and aquatic ecosystems     

Gina E. Hannon  Richard H. W. Bradshaw  Stefan Wastegrd 《第四纪科学杂志》2003,18(7):615-619

High‐resolution pollen, plant macrofossil and sedimentary analyses from early Holocene lacustrine sediments on the Faroe Islands have detected a significant vegetation perturbation suggesting a rapid change in climate between ca. 10 380 cal. yr BP and the Saksunarvatn ash (10 240±60 cal. yr BP). This episode may be synchronous with the decline in δ¹⁸O values in the Greenland ice‐cores. It also correlates with a short, cold event detected in marine cores from the North Atlantic that has been ascribed to a weakening of thermohaline circulation associated with the sudden drainage of Lake Agassiz into the northwest Atlantic, or, alternatively, a period with distinctly decreased solar forcing. The vegetation sequence begins at ca. 10 500 cal. yr BP with a succession from tundra to shrub‐tundra and increasing lake productivity. Rapid population increases of aquatic plants suggest high summer temperatures between 10 450 and 10 380 cal. yr BP. High pollen percentages, concentrations and influx of Betula, Juniperus and Salix together with macrofossil leaves indicate shrub growth around the site during the initial phases of vegetation colonisation. Unstable conditions followed ca. 10 380 cal. yr BP that changed both the upland vegetation and the aquatic plant communities. A decrease in percentage values of shrub pollen is recorded, with replacement of both aquatics and herbaceous plants by pioneer plant communities. An increase in total pollen accumulation rates not seen in the concentration data suggests increased sediment delivery. The catchment changes are consistent with less seasonal, moister conditions. Subsequent climatic amelioration reinitiated a warmth‐driven succession and catchment stabilisation, but retained high precipitation levels influencing the composition of the post‐event communities. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

The angular momentum of gas in protogalaxies – II. The impact of pre-heating     

Frank C. van den Bosch  Tom Abel  Lars Hernquist 《Monthly notices of the Royal Astronomical Society》2003,346(1):177-185

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A field and theoretical analysis of garnet+chlorite+chloritoid+biotite assemblages from the tri-state (MA,CT, NY) area,USA     

Ping Wang  Frank S. Spear 《Contributions to Mineralogy and Petrology》1991,106(2):217-235

The prograde evolution of minerals in metapelites of a Barrovian sequence from the tri-state area (Massachusetts, Connecticut, New York) of the Taconic Range involves assemblages with garnet (Ga), chlorite (Ch), chloritoid (Ct), biotite (Bi) and staurolite (St). Detailed petrologic observations, mineral compositions and chemical zoning in garnet show: (1) garnet high in Mn and Fe but low in Mg is stable with chlorite at grades below those where chloritoid+biotite is found; (2) early formed garnet reacted partially to form Ct+Bi at intermediate grades; (3) at higher grades garnet (with low Mn)+chlorite is again produced, at the expense of chloritoid+biotite, suggesting a reversal in the continuous reaction involving the phases Ga, Ch, Ct and Bi. Thermodynamic modeling of the assemblage Ga+Ch+Ct+Bi±St in the MnKFMASH system reveals: (1) in the MnKFASH system the prograde reaction is Ga+Ch=Ct+Bi whereas in the KFMASH system the prograde reaction is the opposite: Ct+Bi=Ga+Ch; (2) the Ga–Ch–Ct–Bi–St invariant point in the KFMASH system occurs twice, at approximately 6.5 kbar, 545° C and 14.8 kbar, 580° C (although one of them may be metastable in a complex phase system); the appearance of the petrogenetic grid is markedly different from that of Albee, but similar to that of Spear and Cheney; (3) as a consequence, in the KFMASH system, chloritoid+biotite are stable over a wide range of P-T conditions whereas garnet+chlorite assemblages are restricted to a narrow band of P-T conditions; (4) MnO increases the stability field of Ga+Ch relative to both Ct+Bi at low temperatures, and St+Bi at high temperatures; (5) in natural samples the occurrence of Ct+Bi is controlled more by bulk Mg–Fe(-Mn) composition than P-T conditions. Specifically, Ct+Bi is restricted to bulk compositions with Fe/(Mg+Fe+Mn)>0.6. Rocks with Fe/(Mg+Fe+Mn)<0.5 are likely to display only chlorite+biotite at low grade. These observations are consistent with Wang and Spear and Spear and Cheney.  相似文献   

Rigid unit modes in the molecular dynamics simulation of quartz and the incommensurate phase transition     

Frank Stefan Tautz  Volker Heine  Martin T. Dove  Xiaojie Chen 《Physics and Chemistry of Minerals》1991,18(5):326-336

A molecular dynamics simulation of quartz at different temperatures both in the a and in the phase has been conducted. The - phase transition could be observed. A phonon analysis of the -phase confirms and rounds out in a quantitative way the origin of the incommensurate (ic) modulated phase. In particular it traces the optic soft mode at becoming (to a good approximation) a so-called rigid unit mode (RUM) at q0, and elucidates its coupling to the transverse acoustic mode which precipitates the incommensurate transition. This success underpins and illuminates the concept of RUMs and their role in structural phase transitions.  相似文献   

Models of aggradation versus progradation in the Himalayan Foreland   总被引：1，自引：0，他引：1  

Douglas W. Burbank  Richard A. Beck 《International Journal of Earth Sciences》1991,80(3):623-638

A frequent goal of decompaction analysis is to reconstruct histories of basin subsidence and tectonic loading. In marine environments, eustatic and paleobathymetric uncertainties limit the resolution of these reconstructions. Whereas in the terrestrial basins, these ambiguities are absent, it is still necessary to account for depositional slopes between localities in order to analyze three-dimensional patterns of subsidence. We define two end-members for depositional surfaces: aggradation and progradation. The relative importance of either end-member is a function of the interplay between the rate of net sediment accumulation and the rate of basin subsidence. The models predict the patterns of major drainages (transverse versus longitudinal) and the way in which provenance should be reflected within different portions of a basin. Consequently, paleocurrent and provenance data from the ancient stratigraphic record can be used to distinguish between these endmembers. The subhorizontal depositional surfaces that dominate during times of aggradation provide a well defined reference frame for regional analysis of decompacted stratigraphies and related subsidence. Depositional slopes during progradation can not be as precisely specified, and consequently yield greater uncertainties in reconstructions of subsidence. These models are applied to the Mio-Pliocene foreland basin of the northwestern Himalaya, where sequences of isochronous strata have been analyzed throughout the basin. These time-controlled data delineate a distinctive evolution from largely aggradational to largely progradational depositional geometries as deformation progressively encroaches on the foreland. Such a reconstruction of past depositional surfaces provides a well constrained reference frame for subsequent integration of subsidence histories from throughout the foreland.

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Zusammenfassung Ein häufiges Ziel der Dekompaktionsanalyse ist es die Beckenabsenkung und die tektonische Belastung zu rekonstruieren. In marinen Ablagerungsräumen limitieren eustatische und paläobathymetrische Unsicherheiten die Auflösung der Rekonstruktion. Bei terrestrischen Becken fehlen diese Zweideutigkeiten; es ist aber trotzdem notwendig, Rechenschaft über den Ablagerungshang zwischen verschiedenen Lokalitäten abzulegen, um dreidimensionale Subsidenzmuster zu analysieren. Wir definieren zwei Endglieder von Ablagerangsflächen: Aggradation und Progradation. Die relative Wichtigkeit des jeweiligen Endglieds ist eine Funktion des Zusammenspiels zwischen der Nettorate der Sedimentakkumulation und der Beckensubsidenz. Die Modelle sagen die Hauptentwässerungsmuster (quer- oder längsverlaufend) vorher, sowie den Weg in dem die Sedimentherkunft innerhalb verschiedener Bereiche des Beckens berücksichtigt werden sollte. Folglich können Paläoströmungs- und Herkunftsdaten alter stratigraphischer Überlieferungen benutzt werden, um zwischen den Endgliedern zu unterscheiden. Die subhorizontale Ablagerungsfläche welche zur Zeit der Aggradation dominant ist, liefert einen gut definierten Referenzrahmen für die regionale Analyse von dekomprimierten Formationen und der damit verknüpften Subsidenz. Ablagerangshänge während Progradation können nicht präzise spezifiziert werden und beinhalten daher größere Unsicherheiten bei der Rekonstruktion der Subsidenz. Diese Modelle wurden übertragen auf das miozäne bis pliozäne Vorgebirgsbecken des nordwestlichen Himalayas, wo Sequenzen von isochronen Schichten durch das gesamte Becken analysiert werden konnten. Diese zeitkontrollierten Daten schildern eine ganz bestimmte Entwicklung, die von einer hauptsächlich aggradierenden zu einer progradierenden Ablagerangsgeometrie verlief, während der die Deformation schrittweise in Richtung Vorland übergriff. Diese Rekonstruktion von ehemaligen Ablagerangsflächen liefert einen guten Referenzrahmen für die folgende Integration der Subsidenzgeschichte des gesamten Vorlands.

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Résumé L'analyse de décompaction a souvent pour but de reconstituer l'histoire de la subsidence d'un bassin et de la charge tectonique. Dans les milieux marins, de telles reconstitutions sont limitées par des incertitudes de caractère eustatique et paléobathymétrique. Par contre, ces ambiguïtés ne se présentent pas dans le cas des bassins continentaux, où il convient néanmoins de tenir compte de la pente de la surface de dépôt entre les divers points considérés pour établir un schéma tridimensionnel de la subsidence. Nous définissons deux situations extrêmes pour les surfaces de dépôt: l'aggradation et la progradation. L'importance relative de ces deux extrêmes est fonction de l'interaction entre le taux d'accumulation net des sédiments et le taux de subsidence du bassin. Les modèles prévoient la répartition des drainages principaux (transverse ou longitudinal) et la manière dont l'origine des sédiments peut se répercuter dans les diverses parties d'un bassin. Il en résulte que des informations fournies par les relevés stratigraphiques à propos des paléocourants et de la source des sédiments peuvent être utilisées pour faire la distinction entre les deux cas extrêmes. Les surfaces de dépôt subhorizontales, qui prédominent pendant les périodes d'aggradation, fournissent un bon cadre de référence pour les analyses régionales de formations décompactées et de la subsidence qui leur est associée. Les surfaces de dépôt inclinées qui se présentent au cours des progradations ne peuvent pas être définies de manière aussi précise et engendrent par conséquent plus d'incertitude dans la reconstitution de la subsidence. Les auteurs appliquent ces modèles au bassin mio-pliocène d'avant-pays de l'Himalaya nord-occidental, dans lequel des séquences de couches isochrones ont été suivies à travers tout le bassin. Ces données, chronologiquement définies, fournissent l'image d'une évolution nette, depuis des géométries typiques d'aggradation jusqu' à des géométries typiques de progradation, au fur et à mesure de l'emprise progressive de la déformation sur l'avant-pays. Une telle reconstitution des surfaces de dépôt anciennes fournit un bon cadre de référence en vue de l'intégration ultérieure de l'histoire de la subsidence dans l'ensemble de l'avant-pays.

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The influence of mussel beds on nutrients in the Western Wadden Sea and Eastern Scheldt estuaries     

Richard Dame  Norbert Dankers  Theo Prins  Henk Jongsma  Aad Smaal 《Estuaries and Coasts》1991,14(2):130-138

The uptake and release of materials by intertidal mussel beds were directly measured in two cultivated Dutch estuaries. Generally, chlorophylla, seston, and particulate organic carbon were taken up, while ammonium, orthophosphate, and silicate were released. The observed rates were higher than values computed from organismic observations and similar to those observed for intertidal oyster reefs in South Carolina. Specific estuarine material turnover rates varied from 1 week to 38 weeks when calculated with mussel bed fluxes. The fastest turnover rates were for chlorophylla and ammonium. These results support the idea that dense assemblages of bivalves are major components in the recycling of nutrients in estuaries.  相似文献   

V792 Her=HD 155638: A totally eclipsing RS CVn binary     

Charles H. Nelson  Douglas S. Hall  Francis C. Fekel  Robert E. Fried  Richard E. Lines  Helen C. Lines 《Astrophysics and Space Science》1991,182(1):1-17

Photometry of HD 155638=V792 Her has been analyzed to determine the elements of this totally eclipsing RS CVn binary. The light variation outside eclipse was found to have a period of 27^d.07±0^d.07, which is slightly different from the 27^d.5384±0^d.0045 orbital period. Analysis of the eclipses was achieved by a modification of the Russell-Merrill technique. With the aid of radial velocity measures, absolute elements were obtained for the hot and cool stars, respectively;R _h=2.58R ,R _c=12.28R ,M _h=1.40M ,M _c=1.46M ,i=80^o.61 and velocity semi-amplitudesK _c=48.36 km s^–1±0.79 km s^–1, andK _h=50.50 km s^–1±0.33 km s^–1. The apparent magnitudes areV _h=9 ^m .73 andV _c=8 ^m .48. The distance to HD 155638 was estimated to be 310 parsecs.  相似文献   

Variability in the power spectrum of solar five-minute oscillations     

Frank Hill  Juri Toomre  Laurence J. November 《Solar physics》1983,82(1-2):411-425

Two-dimensional power spectra of solar five-minute oscillations display prominent ridge structures in (k, ω) space, where k is the horizontal wavenumber and ω is the temporal frequency. The positions of these ridges in k and ω can be used to probe temperature and velocity structures in the subphotosphere. We have been carrying out a continuing program of observations of five-minute oscillations with the diode array instrument on the vacuum tower telescope at Sacramento Peak Observatory (SPO). We have sought to establish whether power spectra taken on separate days show shifts in ridge locations; these may arise from different velocity and temperature patterns having been brought into our sampling region by solar rotation. Power spectra have been obtained for six days of observations of Doppler velocities using the Mgi λ5173 and Fei λ5434 spectral lines. Each data set covers 8 to 11 hr in time and samples a region 256″ × 1024″ in spatial extent, with a spatial resolution of 2″ and temporal sampling of 65 s. We have detected shifts in ridge locations between certain data sets which are statistically significant. The character of these displacements when analyzed in terms of eastward and westward propagating waves implies that changes have occurred in both temperature and horizontal velocity fields underlying our observing window. We estimate the magnitude of the velocity changes to be on the order of 100 m s^-1; we may be detecting the effects of large-scale convection akin to giant cells.  相似文献